The present invention is in the field of position sensors and position sensor arrangements for contactless position sensing by means of redundant magnet-sensitive sensor elements.
Such an arrangement can in particular be used in connection with a “shift-by-wire” system for determining the command by the driver in case of an automatic transmission of a motor vehicle. The command by the driver determines the functional behavior of the transmission. Determining the command from the driver in connection with an automatic transmission comprises typically the following driving modi selection: parking, reverse driving, neutral and driving forward. Optionally driving forward is divided in two or more driving positions. Determining the command intended by the driver is crucial for proper operation of the vehicle and is therefore relevant as to safety. Therefore a safe determination has to be guaranteed and possible sensor errors have to be detected. In some applications further single error redundancy is required enabling limited emergency running properties.
Conventional prior art automatic transmissions comprise in addition to an electrical sensing of the driver command a mechanical connection to the hydraulic control of a automatic converter. The determination by means of a sensor and the electrical control plus the mechanical locking device represent a redundant system preventing the electronic sensor from determining, in the event of an error, the driving conditions. With such automatic transmissions the electronic sensor sensing the driver command is not to the same extent relevant as to safety.
However, in case of a “shifted-by-wire” system such a mechanical connection to the transmission is missing. The driver's command is sensed exclusively by means of electronic sensors and the appropriate actions by the transmission are conducted due to the information from the transmission selector switch (transmission range sensor) by means of electrical actuators in the transmission. An alternative form of the transmittal of this information that would allow locking in the case of an error does not exist in this case. Thus, the driver command sensing requires a very high reliability of the electronic sensor system.
In case of typical shift-by-wire-systems, the driver's command is transmitted at first mechanically by means of a push pull cable. The conveyed linear movement is converted by means of a lever into a rotational motion. The rotational motion includes a rest position corresponding with the appropriate drive positions. A position sensor arrangement is adapted to determine this angle of rotation. In particular, in the areas of the rest positions, a good detection is required.
Detection of the rotational angle by means of a sensor can be achieved in the various manners. In the automotive field, magnetic concepts dominate as robust systems that are cost efficient. Typically, these comprise pivoted magnetic transmitters interacting with a stationary sensor sensing the magnetic field. The transmitter is usually designed as a permanent magnet. For the actual sensing process, a characteristic component of the magnetic field is measured that is in a defined dependency on the parameter to be measured, namely the rotational angle. The advantage of this measuring method is that it does work contactless and therefore avoids problems due to contact corrosion or wear. A disadvantage is, however, that such sensors are generally sensitive to errors caused by external magnetic fields.
From the prior art systems are known where the transmitter comprises a homogeneous magnetic field and a sensor unit that transmits a signal dependent on the angle of the lines of the magnetic field. These systems can be made redundant by duplicating of the sensor unit. A disadvantage of such systems that provide only redundancy at the end of the sensor is the sensitivity against magnetic influences as, for instance, caused by electric current conductors since a change in the rotational angle of the magnetic field caused by undesired magnetic field interference cannot be distinguished from an actual change in the rotational angle of the transmitter. By means of a further magnetic transmitter having an orientation of its lines of the magnetic field preferably at about 90 degrees in relation to the first magnetic transmitter and that is coupled with the first magnetic transmitter is generally capable to distinguish an actual change in the rotational angle from a virtual change of the angle caused by any undesired magnetic interference. In order to fulfill the requirements as to reliability and to provide a single redundancy, this concept requires three sensor units and three transmitter units, i.e. magnets. This results in a non-acceptable size. A further disadvantage results from the fact that the additional measurement levels, due their distance from each other, are not subjected necessarily to the same interference vector, making a correction even more difficult.
From DE 199 56 361 C2, an angle of rotation sensor for sensing the angle of rotation in a homogeneous magnetic field is known. Although in connection with this sensor the arrangement of redundant magnet sensors is suggested, as a matter of principle it is impossible due to the homogeneous field to recognize interference fields under all possible conditions reliably and to minimize their influence.